What happens when your book collection outgrows your home? One San Francisco couple decided to open a library and share it with the public for free.

"This is an extremely large collection," said Rick Prelinger, one of the co-founders of the Prelinger Library.

If libraries are repositories for thoughts, the Prelinger Library will make you think like never before. It is a private collection of "this" and "that" stacked to the ceiling.

Rick and Megan Prelinger have spent decades collecting old maps, advertising magazines, and technical handbooks. They call it historical ephemera.

"We're not good on sports, we're not good on nursing, we have relatively little on fashion," said Rick. "We collect what we like."

They started in collecting in 1982. When the collection outgrew their home, they moved to San Francisco's South of Market neighborhood. Some of the books were purchased, others were donations or came from old libraries. It is all carefully curated to meet the Prelinger's hope of preserving the past.

"We were both very interested in what kind of material wasn't in a every library, what falls between the cracks, what can you find in odd places and what kind of picture of history would you get if you put all that together," said Megan.

The Prelinger Library is full of things that most libraries would have tossed out. It includes 30,000 bound objects, 60,000 loose sheets of paper, and 10,000 magazines.

Showing off one of their prized books, Megan beamed, "To show a food futurist the volume of the Candy Industry Journal can formatively blow their mind."

The library is open to the public and free. It's funded by donations. Because many of the copyrights have expired, visitors are welcome to copy anything in the collection.

"You know we love evidence, and if you look at an old magazine, or an old pamphlet, and even an old government document it's filled with ideas that might not hold up today, but historically they are incredibly valuable," said Rick.

The Prelingers are digitizing the library as well making much the collection available online.

"We hold onto things and make things available that are really not digitizable," added Megan.

The Prelingers also collected old films, everything travel guides to commercials, instructional and educational films, and even corporate promotional films.

The library gave that treasure trove of films away so they could be researched and enjoyed by the world for free.

"That collection went to the Library of Congress in 2002, 18 tractor trailer loads," said Rick.

Those films are still available on the Prelinger website, joining Rick's latest obsession, home movies.

"Home movies are personal documents. It's people shooting things they care about, people they love, things that mean a great deal to them. And home movies, it turns out, is the great hidden and unknown record of 20th century life," said Rick.

So why would someone save all of this? The Prelingers say it's all about keeping the past alive.